A Four Gain Readout Integrated Circuit

The main difficulty for the readout electronics of the ATLAS LARG calorimeter is to handle the 16 bit dynamic range without spoiling the signal to noise ratio. A possible way out is to split the input signal in four « ranges » with different gains and to select the most appropriate. Then, each subrange only requires 10 bits, a much more manageable number. A four gain shaper circuit has been successfully developed at LAL Orsay and we report in the following about the associated « gain selector » circuit. It is a synchronous design running with the 40MHz LHC clock and using a fully differential architecture to minimise pick-up from digital activity. The AMS 1.2 micron bipolar technology used should provide enough radiation hardness (200Gy are expected over 10 LHC years). After a description of the content of the four gain selector circuit, some preliminary results from the first prototype will be presented.